The objective of this project is to establish the relative importance of liver and muscle in the metabolism of the branched-chain amino acids and to determine the relationship between alterations in the activities of enzymes for the degradation of histidine and phenylalanine and the physiological capacity of the intact animal to degrade these amino acids. Isolated perfused rat hindquarter will be used initially to determine the extent to which this preparation, which consists largely of muscle, degrades the branched-chain amino acids to CO2 and to branched-chain alpha-keto acids. Concentrations of leucine, isoleucine and valine in the perfusion medium will be increased gradually to determine the concentration at which branched-chain alpha-keto acids begin to accumulate in the perfusion medium. Intact animals will be administered, orally or intraperitoneally, loads of the branched-chain amino acids comparable to those used in the in vitro studies to determine the relationship between the load administered and the concentrations of branched-chain alpha-keto acids in plasma. In both types of preparations effects of pretreatment of the animals with hormones such as glucagon and insulin, and with different diets, all of which are known to influence amino acid transport and metabolism on the accumulation of both animo and keto acids, will be examined. For studies of effects of adaptive responses of histidine and phenylalanine degrading enzymes on the capacity of animals (rats) to degrade these amino acids, animals will be subjected to nutritional or hormonal treatment that alter the activities of histidase, histidine and phenylalanine amino transferases and decarboxylases over a wide range. Subsequently the ability of animals to degrade the appropriate amino acids will be examined. This will be done by determining the capacity of the rat to degrade specifically labelled phenylalanine and histidine to carbon dioxide and by estimating quantitatively urinary excretion of various metabolites.